Wind turbines, like all structures, can interfere with communication or radar signals when these signals are interrupted by the turbine structure or the rotor plane. Wind turbines can sometimes cause electromagnetic interference affecting TV and radio reception. Electromagnetic interference can be caused by near-field effects, diffraction, or reflection and scattering. Such interference can typically be mitigated by using satellite TV or wireless cable TV. Although instances of TV or radio interference are infrequent and typically straightforward to mitigate, the interaction of wind turbines and navigational or defense radar signals is the subject of considerable recent attention.
Interference with Navigational and Defense Radar
Navigational and defense radar interference is an issue that needs to be addressed by wind developers. In the majority of cases, interference is either not present, is not deemed significant, or can be readily mitigated. Understanding the extent of a wind installation's radar interference potential and developing mitigation techniques can be more complicated than for other forms of potential interference, as it depends on turbine height, rotor sweep area, blade rotation speed, and the landscape surrounding a wind energy project.
Types of Interference
Wind turbines, like other large, metallic structures — such as buildings, TV towers, and satellite dishes — are radar reflectors, and as such, all of these types of structures have the potential to cause radar interference if placed in sensitive locations. There are two types of interference: direct interference and Doppler interference. Direct interference happens with high reflectivity and reduces radar sensitivity, sometimes producing false images ("ghosting") or shadow areas ("dead zones"). Doppler interference creates false targets and impacts both airborne and fixed radar.
Federal Aviation Administration
As summarized by the Airspace Issues in Wind Turbine Siting Web page maintained by the Massachusetts Technology Collaborative, "The FAA has oversight of any object that could have an impact on the navigable airspace or communications/navigation technology of aviation (commercial or military) or Department of Defense (DOD) operations. The FAA requires that a Notice of Proposed Construction (Form 7460-1) be filed for any object that would extend more than 200 feet above ground level (or less in certain circumstances, for example if the object is closer than 20,000 feet to a public-use airport with a runway more than 3,200 feet long). As wind turbine heights have increased during the past couple of decades, this filing requirement has applied to increasing numbers of projects."
"For any filed project, the FAA undertakes an initial aeronautical study within the relevant FAA region, and issues either a Determination of No Hazard to Air Navigation (DNH) — the "green light" for the project — or a Notice of Presumed Hazard (NPH). If an NPH is issued, the FAA will then initiate an in-depth technical analysis (commonly called an extended study), which will explain the cause of the NPH and evaluate impacts on air operations. If after the extended study, which may include a public comment period, there remains an operational impact, the FAA will try to negotiate an acceptable height for a project that has received a NPH. If no agreement can be reached, FAA will issue a Determination of Hazard (DOH). A DOH can be appealed to FAA Washington Headquarters. If the appeal does not secure a DNH, the proponent's main recourse is to bring the issue before a Federal Court."
There have been relatively few cases in which the FAA retroactively determined that a wind farm was interfering with radar equipment, thus requiring the developer to make changes such as moving individual turbines or changing the turbine heights. Such mitigation efforts can trigger unanticipated cost for developers, which can retroactively impair a project's financial viability. As a result, wind developers have sought clearer up-front estimates of potential mitigation costs or a stronger guarantee of FAA approval. Because of the complex nature of radar interference that is affected by many factors other than turbine location and height, the FAA cannot provide a guarantee that a wind farm will not interfere with radar. However, the FAA has stated that it will not halt a project that has begun construction, and developers should be confident in proceeding with construction once they receive a DNH letter from the FAA.
In most cases, radar interference can be corrected with software that deletes radar signals from stationary targets. The Middlegrunden Wind Farm in Denmark is located just 8 miles from the Copenhagen airport, but all airport computers have software that corrects for radar signals from the wind farm. Most turbine and radar interaction problems concerning the FAA can be addressed using available software upgrades.
Department of Defense
In addition, the Department of Defense (DOD) often tracks objects (some of which may be rather small) that might be blocked, or whose characteristics may be distorted or displaced, by turbine interference. For such instances, software fixes alone may be insufficient to resolve potential interference with military radar applications, so additional mitigation techniques may be required. Tests indicate that turbines reduce the probability of radar detections in some cases. Of course, not all cases of turbine-radar proximity lead to interference. Some Air Force runways with turbines located just miles away experience no interference problems.
The National Defense Authorization Act for Fiscal Year 2006 included an amendment requiring DOD to study and report on the effects of wind projects on military readiness. The report concluded that "Given the expected increase in the U.S. wind energy development, the existing siting processes as well as mitigation approaches need to be reviewed and enhanced in order to provide for continued development of this important renewable energy resource while maintaining vital defense readiness. The Department of Defense strongly supports the development of renewable energy sources and is a recognized leader in the use of wind energy." It went on to acknowledge that while wind turbines located in radar line of sight of air defense radars can adversely impact their ability to carry out their national defense mission, the magnitude of the impact will vary. Mitigation tools that currently exist to completely preclude any adverse impacts on air defense radars are limited and require case-by-case analysis. However, DOD is committed to developing additional mitigation approaches.
See: U.S. Department of Defense. (2006). "The Effect of Windmill Farms On Military Readiness (PDF 1.3 MB) Download Adobe Reader."
A number of tools and practices are available to manage or mitigate the potential impact of wind turbine interference.
- Conducting studies to ensure that the wind farm location is not in an area of high radar activity. Studies should also analyze the potential interference effects of the individual turbines and the wind project as a whole. Farm layout optimization, terrain masking, or reduction of the radar cross-section area may be sufficient to address identified interference problems.
- Coating equipment with absorbent or reflective materials to minimize the turbine's radar signature.
- Starting early communications between wind developers and the potentially affected federal agencies, such as the FAA and the DOD, to mitigate potential radar interference. Often the easiest and least costly approaches involve software optimization. Other options include installing post-processors or adding hardware (such as processors, transmitters, or receivers). When such changes alone are insufficient, more involved approaches can sometimes be implemented. These include deploying extra radars to cover the shadow spots, relocating radar installations to accommodate the new wind farms, or altering air traffic routes around new wind farms.
Even with these mitigation methods, there will be some proposed locations where wind turbines will cause disruptive radar interference. In such cases, wind projects would likely be unable to proceed at the proposed site.
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